1. Field of the Invention
This invention relates in general to oil and gas wellhead assemblies, and in particular to an annulus seal for sealing between a casing hanger and a bore of a wellhead housing.
2. Description of the Prior Art
In wells of the type concerned herein, a wellhead housing will be located at the upper end of the well. The wellhead housing is a large tubular member having an axial bore extending through it. Casing will extend into the well and will be cemented in place. A casing hanger, which is on the upper end of the casing, will land within the wellhead housing. The exterior of the casing hanger is spaced from the bore of the wellhead housing by an annular clearance which provides a pocket for receiving an annulus seal.
There are many types of annulus seals, including rubber, rubber combined with metal, and metal-to-metal. One metal-to-metal seal in use has a U-shape, having inner and outer walls or legs separated from each other by an annular clearance. An energizing ring, which has smooth inner and outer diameters, is pressed into this clearance to force the legs apart to seal in engagement with the bore and with the exterior of the casing hanger. Wickers are located on the exterior of the casing hanger and in the bore of the wellhead housing. The outer leg embeds into the wickers of the bore while the inner leg embeds into the wickers of the casing hanger. This locks the annulus seal in place.
Temperature cycles can cause the casing hanger to move axially short distances relative to the wellhead housing. The temperature cycles are due to the production fluids flowing through the casing, the temperature cycles resulting in elongation and contraction of the casing. Because the inner and outer legs are embedded into wickers, the inner leg will tend to move with the axial movement of the casing hanger. The outer leg will remain stationary with the wellhead housing. Although unlikely, this slight relative axial movement between the inner and outer legs can conceivably cause the energizing ring to work upward from its energized position.
The U-shaped metal-to-metal seal described above works very well and does not deteriorate over many years of usage. However in some installations, the pressure expected to be encountered is not particularly high. In those installations, the cost of a metal seal may not justify the expected pressures. The prior art style rubber seals may deteriorate more quickly than desired.